A Japanese friend of mine claimed that Allied forces used poisonous gas during the Battle of Okinawa. I asked for a link to such a claim, and got a link to this section of the Japanese language Wikipedia article on the Battle of Okinawa.

Did Allied forces use chemical weapons in Okinawa during World War II?

I'm skeptical because I hadn't heard about Allied forces (especially non Soviet Union Allied forces) using chemical warfare in World War II, and because denial of Japanese atrocities, and false claims of Allied atrocities, is fairly mainstream in Japan.

While searching for information on this, I came across pages stating that after the war, the United States stored chemical weapons in Okinawa. I am not asking about this in this question.

2 Answers
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Okinawans, including honorable civic leaders who weren't anti-American, [charged] that the poison gas [was] used on some caves was in violation of the Geneva Convention. Well after the war, still haunted by the huge number of civilian deaths, they'd ask interested Americans to investigate. They did, and discovered what actually happen: American explosives unintentionally released poison gases that cause thousands of agonizing deaths in confined cave spaces. The most lethal was the yellow smoke of the petric acid [sic] used in Japanese munitions stored there.

However, there is no such thing as "petric acid", perhaps he means picric acid.

Picric acid smoke can be yellow, that is true. I'm not sure it's very poisonous or most lethal though. In fact "most lethal" does not sound like a desirable characteristic of any propellant residue. Combustion products (CO, CO2, no O2) can sadly be deadly of itself.
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ChrisWJun 2 at 18:53

@ChrisW There was also a mass picric acid poisoning of US sailors on a ship anchored at Wakayama, Japan in 1946, due to traces in drinking water. No fatalities, but more than 100 with blood in urine. I don't know full details. see page 239 here books.google.com/… so it could be picric acid itself too.
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DavePhDJun 2 at 19:18

The CDC warns of kidney problems, skin irritations, swallowing causes nausea etc., high doses attack blood cells and damage the liver, and yes, blood in the urine ... I think any "yellow" smoke from picric acid is uncombusted/exploded rather than detonated.
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ChrisWJun 2 at 19:23

I mean, +1 for supplying a reference as you did, but fwiw I'm not sure how/whether that last sentence in the quote is true (or if it's true, complete).
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ChrisWJun 2 at 19:26

Battle of Okinawa, 1945

The weapons used include

flame-throwers

chemical smoke mortar rounds

white phosphorus grenades

An hour later another infantry assault was attempted, supported by 105-mm. artillery, light tank fire, antitank guns, heavy machine guns, 60-mm. and 81-mm. mortars, 4.2-inch chemical mortars, and bazookas; but the attack was again stopped by Japanese who hid underground during the heavy fire and then rushed back to their firing positions to meet the oncoming Americans.

It is unclear whether the chemical mortar rounds were designed merely to produce smoke to hide troop advances or were intended to harm enemy troops. From the text it seems they were probably intended to produce smoke through which US troops could move without being seen.

The 4.2-inch chemical smoke on the south side of Kakazu West was interspersed with high explosive artillery shells to keep the enemy pinned down. Under cover of the smoke the survivors of Company L pulled back off the hill to the gorge, carrying their wounded with them.

As well as flamethrowers and "chemical smoke", according to this account, the US army used phosphorus grenades

Colonel Maybury directed supporting fire in front of Company C, which quickly moved to the top without losing a man. It then proceeded leisurely and methodically to destroy the remaining Japanese with white phosphorus grenades and flame throwers. Only 20 of the 110 defenders escaped to the south.

It isn't 100% clear but this account can be interpreted as meaning that phosophorus grenades were used in an anti-personnel role.

The account doesn't identify the type of grenade, A more recent type is described as

The M15 White Phosphorous grenade is a bursting type grenade used for signaling, screening, and incendiary purposes.

The M34 chemical smoke grenade is the most versatile of all hand grenades. The grenade can be used for signaling, screening, or incendiary missions, or for producing casualties ... The filler has 15 ounces of white phosphorous.

Geneva Protocol, 1925

The 1925 Geneva Protocol prohibits the use of chemical and biological weapons in war. The Protocol was drawn up and signed at a conference which was held in Geneva under the auspices of the League of Nations from 4 May to 17 June 1925, and it entered into force on 8 February 1928.

...

a sizeable fraction of its parties have reserved to themselves a right to retaliate in kind if chemical and/or biological weapons should ever be used against them by enemies or allies of enemies.

Chemical Weapon

The general and traditional definition of a chemical weapon is a toxic chemical contained in a delivery system, such as a bomb or shell.

The Convention defines chemical weapons much more generally. The term chemical weapon is applied to any toxic chemical or its precursor that can cause death, injury, temporary incapacitation or sensory irritation through its chemical action. Munitions or other delivery devices designed to deliver chemical weapons, whether filled or unfilled, are also considered weapons themselves.

The toxic chemicals that have been used as chemical weapons, or have been developed for use as chemical weapons, can be categorised as choking, blister, blood, or nerve agents.

However this is a modern definition and the Convention referred to is a more recent one.

Chemical Weapons Convention 1992

Although produced 47 years after the date of the events in the question, this convention defines chemical weapons and so has some relevance.

1991 US View re phosphorus

POSSIBLE USE OF PHOSPHOROUS CHEMICAL WEAPONS BY IRAQ IN KURDISH AREAS ALONG THE IRAQI-TURKISH-IRANIAN
BORDERS

Some commentators have suggested that this phrase illustrates that WP is regarded as a chemical weapon by US intelligence.

Conclusion

According to the account given above, the US army did use flamethrowers, chemical smoke and white phosphorus grenades. These were probably not considered chemical weapons by the standards of the time. Nor by a strict interpretation of current conventions.

Speculation: A soldier defending his homeland against foreign invaders might take a different view if the invaders are using flamethrowers and white phosphorus grenades against him or her. Perhaps, literally in the fog of war, the unpleasant and potentially harmful smoke from white phosphorus grenades was described by Japanese soldiers as "poison gas".

They also used CS(also known as tear) gas which while not normally deadly is an irritant and would fall into the category of chemical weapons though it is not in the prohibited class. As of 2007 I know the US ARMY used CS gas in basic training.
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ChadJan 17 '13 at 18:18